9.4: Multiplication of Polynomials by Binomials
Suppose a factory needs to increase the number of units it outputs. Currently it has \begin{align*}w\end{align*}
Watch This
Multimedia Link: For further help, visit http://www.purplemath.com/modules/polydefs.htm – Purplemath’s website – or watch this CK12 Basic Algebra: Adding and Subtracting Polynomials
YouTube video.
Guidance
A binomial is a polynomial with two terms. The Distributive Property also applies for multiplying binomials. Let’s think of the first parentheses as one term. The Distributive Property says that the term in front of the parentheses multiplies with each term inside the parentheses separately. Then, we add the results of the products.
\begin{align*}(a+b)(c+d)=(a+b)\cdot c+(a+b)\cdot d\end{align*}
We see that we can apply the Distributive Property on each of the parentheses in turn.
\begin{align*}c \cdot (a+b)+d\cdot (a+b)=c\cdot a+c \cdot b+d \cdot a+d \cdot b \ (\text{or} \ ca+cb+da+db)\end{align*}
What you should notice is that when multiplying any two polynomials, every term in one polynomial is multiplied by every term in the other polynomial.
Example A
Multiply and simplify \begin{align*}(2x+1)(x+3)\end{align*}
Solution: We must multiply each term in the first polynomial with each term in the second polynomial. First, multiply the first term in the first parentheses by all the terms in the second parentheses.
Now we multiply the second term in the first parentheses by all terms in the second parentheses and add them to the previous terms.
Now we can simplify.
\begin{align*}(2x)(x)+(2x)(3)+(1)(x)+(1)(3) & = 2x^2+6x+x+3\\
& = 2x^2+7x+3\end{align*}
Example B
Multiply and simplify \begin{align*}(4x5)(x^2+x20)\end{align*}
Solution:
Multiply the first term in the binomial by each term in the polynomial, and then multiply the second term in the monomial by each term in the polynomial: \begin{align*}(4x)(x^2)+(4x)(x)+(4x)(20)+(5)(x^2)+(5)(x)+(5)(20)&=4x^3+4x^280x5x^25x+100\\
& = 4x^3x^285x+100\end{align*}
Solving RealWorld Problems Using Multiplication of Polynomials
We can use multiplication to find the area and volume of geometric shapes. Look at these examples.
Example C
Find the area of the following figure.
Solution: We use the formula for the area of a rectangle: \begin{align*}\text{Area}=\text{length}\cdot\text{width}\end{align*}
\begin{align*}\text{Length} & = B+3, \ \text{Width}=B+2\\
\text{Area} &= (B+3)(B+2)\\
& = B^2+2B+3B+6\\
& = B^2+5B+6\end{align*}
Guided Practice
Find the volume of the following figure.
Solution:
\begin{align*}The \ volume \ of \ this \ shape & = (area \ of \ the \ base) \cdot (height).\\
\text{Area of the base} & = x(x+2)\\
& = x^2+2x\end{align*}
\begin{align*}Volume&=(area \ of \ base ) \times height\\
Volume&=(x^2+2x)(2x+1)\end{align*}
Now, multiply the two binomials together.
\begin{align*}Volume&=(x^2+2x)(2x+1)\\
&= x^2\cdot 2x+x^2\cdot 1+2x\cdot 2x+ 2x\cdot 1\\
&= 2x^3+x^2+2x^2+2x\\
&=2x^3+3x^2+2x\end{align*}
Practice
Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK12 Basic Algebra: Multiplication of Polynomials (9:49)
Multiply and simplify.

\begin{align*}(x2)(x+3)\end{align*}
(x−2)(x+3) 
\begin{align*}(a+2)(2a)(a3)\end{align*}
(a+2)(2a)(a−3) 
\begin{align*}(4xy)(2x^4 yz^3 y^4 z^9)\end{align*}
(−4xy)(2x4yz3−y4z9) 
\begin{align*}(x3)(x+2)\end{align*}
(x−3)(x+2) 
\begin{align*}(a^2+2)(3a^24)\end{align*}
(a2+2)(3a2−4) 
\begin{align*}(7x2)(9x5)\end{align*}
(7x−2)(9x−5) 
\begin{align*}(2x1)(2x^2x+3)\end{align*}
(2x−1)(2x2−x+3) 
\begin{align*}(3x+2)(9x^26x+4)\end{align*}
(3x+2)(9x2−6x+4) 
\begin{align*}(a^2+2a3)(a^23a+4)\end{align*}
(a2+2a−3)(a2−3a+4) 
\begin{align*}(3m+1)(m4)(m+5)\end{align*}
(3m+1)(m−4)(m+5)
Find the areas of the following figures.
Find the volumes of the following figures.
Mixed Review
 Simplify \begin{align*}5x(3x+5)+11(7x)\end{align*}
5x(3x+5)+11(−7−x) .  Cal High School has grades nine through twelve. Of the school's student population, \begin{align*}\frac{1}{4}\end{align*}
14 are freshmen, \begin{align*}\frac{2}{5}\end{align*}25 are sophomores, \begin{align*}\frac{1}{6}\end{align*}16 are juniors, and 130 are seniors. To the nearest whole person, how many students are in the sophomore class?  Kerrie is working at a toy store and must organize 12 bears on a shelf. In how many ways can this be done?
 Find the slope between \begin{align*}\left ( \frac{3}{4},1 \right )\end{align*} and \begin{align*}\left ( \frac{3}{4}, 16 \right )\end{align*}.
 If \begin{align*}1 \ lb=454 \ grams\end{align*}, how many kilograms does a 260pound person weigh?
 Solve for \begin{align*}v\end{align*}: \begin{align*}16v=3\end{align*}.
 Is \begin{align*}y=x^4+3x^2+2\end{align*} a function? Use the definition of a function to explain.
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Term  Definition 

binomial  A binomial is a polynomial with two terms. 
Distributive Property for Binomials  To use the Distributive Property with two binomials, multiply each term in the first factor by each term in the second. 
distributive property  The distributive property states that the product of an expression and a sum is equal to the sum of the products of the expression and each term in the sum. For example, . 
Image Attributions
Here you'll learn how to multiply a polynomial by a binomial.